1. Field of the Invention
The present invention relates to a probe control and measurement data correction technique which addresses adhesion of a probe to a side wall of a measured pattern in measurements using a scanning probe microscope.
2. Description of the Related Art
Scanning probe microscopes (SPM) are known as one of the techniques for measuring a microscopic 3D shape. In this technique, a sharp-pointed probe is brought into proximity to or contact with the surface of a sample, and the measured amount of physical interaction such as an atomic force or the like being generated between the probe and the sample at this time is displayed as an image. An atomic force microscope (AFM), which is one of the SPM, is a technique in which an atomic force acting between a probe attached to the end of a beam supported at only one end (leaf spring) called a cantilever and a sample, namely, a contact force between the probe and the sample is detected by the amount of flexure of the cantilever, and the sample surface is scanned while performing control such that the amount of flexure is kept constant, to measure a microscopic shape on the sample surface. The AFM is widely used in various fields such as biology, physics, semiconductors, storages or the like as the technique allowing to measure a microscopic 3D shape in an atomic order.
Especially in the fields of semiconductors and storages, patterns have been increasingly reduced in size every year, and thus, there is a growing expectation for the AFM as the technique allowing to measure a microscopic 3D shape in an atomic order.
Some methods are generally known as the AFM measuring method, and the methods are selectively used depending on a measurement purpose. For example, contact mode, which is most common, is a method of performing scanning with a probe continuously contacting a sample, and is mainly suitable for measuring a flat sample.
On the other hand, in order to measure a pattern having a high aspect ratio, the following measuring methods suitable for the high aspect pattern are used. One of the methods is a method called cyclic contact mode, in which a probe is vibrated at a vibration frequency near a resonance point, and scanning is performed while performing control such that the vibration amplitude (setpoint) of the probe at the time when the probe contacts a sample is kept constant. In this method, damages to a soft and brittle sample and the probe are reduced since the probe intermittently contacts the sample during probe scanning and the probe does not continuously drag along the sample as in the contact measurement.
Also, a method disclosed in Japanese Patent Publication No. 3925380 (Patent Document 1) can be cited as an effective measuring method in measuring the high aspect pattern. This method is a method of performing a measurement by completely pulling a probe away from a sample 101, moving the probe to a next measuring point, and then, bringing the probe closer to the sample again during probe scanning. Both of the above measuring methods are featured by bringing the probe into intermittent contact with the sample.
In the case of the cyclic contact mode method, the measurement is performed with the probe dynamically contacting the sample, unlike the method disclosed in Patent Document 1 in which the measurement is performed with the probe statically contacting the sample. Moreover, Japanese Patent Laid-Open Publication No. 2004-132823 (Patent Document 2) discloses a measuring method in which a method disclosed in Japanese Patent Laid-Open Publication No. 11-352135 (Patent Document 3) and the probe operation disclosed in Patent Document 1 are combined (a method of performing a measurement by repeating retraction from and approach to a sample with a probe being vibrated).
Also, Japanese Patent Laid-Open Publication No. 2006-329973 (Patent Document 4) discloses a method of controlling all the time a retraction position of a probe at a minimum required distance to be released from adhesion and temporarily increasing the retraction distance when a bump is detected, and also, an example in which a carbon nano tube is bent by an electrostatic force to accurately measure the bump. Japanese Patent Laid-Open Publication No. 07-270434 (Patent Document 5) discloses that a probe capable of vibrating in a horizontal direction is brought into contact with a side wall by successive taps such that the probe is not affected by adhesion when the probe contacts the side wall of a groove in an integrated circuit. Also, a method of removing a measurement error resulting from a probe shape is described in J. S. Villarrubia, “Algorithms for Scanned Probe Microscope Image Simulation, Surface Reconstruction and Tip Estimation”, Journal of Research of the National Institute of Standards and Technology, Volume 102, Number 4, July-August 1997 (Non-Patent Document 1), and David Keller, “Reconstruction of STM and AFM images distorted by finite-size tips”, Surface Science, 253 (1991) 353-364 (Non-Patent Document 2).
Among the fields in which the AFM is increasingly used, circuit patterns have been increasingly reduced in size along with circuit integration in the field of semiconductors, and recording patterns have been increasingly reduced in size as recording densities have been increased in capacity in the field of optical disks and magnetic disks (patterned media). It is desirable that patterns are evaluated in a development process and the results are fed back to the development process to thereby improve the efficiency of development. The size reduction of the measured patterns (semiconductor circuit patterns, optical disk pits, bit arrays of patterned media) reaches the order of several tens of nm. Thus, when these patterns are measured by the AFM, a technique for handling an elongated probe used for the pattern having a high aspect ratio becomes important.
As a typical elongated probe, there are an Si probe whose tip is sharp-pointed for the pattern having a high aspect ratio, a carbon nano tube (CNT) probe as a carbon probe, a high density carbon (HDC) probe or the like. Among the above probes, the Si probe and the HDC probe have a tip diameter of about a few nm at the smallest, but have a tapered shape, and the aspect ratio is 10 or more at the largest. On the other hand, the CNT probe used in the AFM has a probe diameter of 10 nm or more, but has a columnar shape, and the aspect ratio is higher. Accordingly, the CNT probe is very useful for measuring a microscopic pattern having a high aspect ratio.
However, in the case where a pattern having a steep side wall is measured using the elongated probe, there occurs a problem that, when the probe approaches the side wall of the pattern, the probe is attracted and adheres to the side wall due to the van der Waals forces acting between the probe and the side wall. An error occurs in the measured profile of the side wall portion due to torsion of a cantilever and deflection of probe caused by the adhesion. Furthermore, in the case where a pattern having a groove width almost equal to the diameter of a probe is measured, the probe adheres to both side walls and the probe cannot reach the groove bottom. Thus, the groove depth cannot be measured. Also, if a measurement contact force is set to be large such that the probe reaches the groove bottom, there occurs a new problem that damages to the probe and the sample are increased and a profile error increases due to slipping of the probe occurring in the steep side wall portion of the pattern.